Have you ever heard of "dark lightning?" Few people outside the scientific community have, but it is something real that is actually quite powerful - and possibly dangerous. A group of scientists in Florida has been learning about this mysterious natural phenomenon:

We all know what thunderstorms are, and how much havoc their violent winds, torrential rains and lightning strikes can cause. But over the past 10 years, scientists have learned of an even darker side to thunderstorms: they can generate powerful bursts of electromagnetic energy known as Terrestrial Gamma-Ray Flashes, or TGFs.

“A few years back, a spacecraft started seeing these bursts of gamma rays coming up from the Earth’s atmosphere," said Joseph Dwyer. "It was very strange. The Earth is not supposed to make gamma rays. If you want to study gamma rays you usually look for places like black holes and supernovas. We figured out eventually that these gamma rays were coming from ordinary thunderstorms.”

Professor Joseph Dwyer and his colleagues at the Florida Institute of Technology have been researching so-called “dark lightning” for several years. Dwyer says that while the phenomenon is quite different from what we see flashing brightly in the sky during a thunderstorm, the two types of high-energy events can be produced by the same storms, but in different ways.

“Normal lightning is very hot," he said. "It’s about five times as hot as the surface of the sun and because of that emits a lot of light. But, compared to the gamma ray energy scale, it’s downright cold. So normal lightning is not hot enough to make the kind of gamma rays we’ve been seeing and so we needed some other explanation. What we now think is going on is that a thunderstorm acts like a gigantic particle accelerator. Strong fields inside the thunderstorm accelerate electrons to almost the speed of light and then they make the gamma-rays.”

A tremendous amount of energy is released in dark lightning, yet its powerful discharge is silent, and almost completely invisible to the unaided eye.

Scientists have been concerned that since these gamma-ray bursts can originate at the same altitudes where commercial aircraft fly, they could damage the planes and jeopardize the safety of airline passengers. But Dwyer points to a couple of factors that minimize those dangers.

“First of all, pilots do their best to stay away from thunderstorms," said Dwyer. "Thunderstorms are dangerous places; we all know that already, so no additional warning is needed. And the second piece of good news is dark lightning appears to be relatively rare, maybe one out of every thousand normal lightning flashes would be dark lightning. So combining those two, people should not be worrying about this.”

Dwyer notes that astronauts peering down from Earth-orbiting spacecraft have reported that these gamma-ray producing storms occur most often around the equatorial regions of the planet. Dwyer says that could be because storms in those areas tend to be taller, higher-altitude thunderstorms, so their gamma-rays are bursting closer to space - and more visibly to the astronauts - since there’s less atmosphere for the light to pass through.

Dwyer says that in general, any thunderstorm should be capable of generating dark lightning. He says he and his colleagues are still not certain what’s happening inside a thunderstorm that makes one storm more likely than another to generate the gamma-ray discharges, so more research on dark lightning is needed.

“It would be very nice to have instruments that were specifically designed to measure what we’re interested in studying," he said. "Now, we’re talking about something that’s happening right over our heads that could affect people, that may be relatively common and so it would be very interesting to learn more about this.”

The researchers say new data from special Earth-observing satellites will help them better understand dark lightning. And while studies of the phenomenon continue, Professor Dwyer’s research has found no evidence yet that the mysterious gamma-ray bursts in thunderstorms pose any direct threat to public health or the environment.

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